1. Field of the Invention
This invention relates to a programmable waveform generating system especially adapted to produce strings of output pulses suitable for measuring the response of biological samples to electrical stimulation under different conditions. More particularly, it relates to such a system suitable for measuring responses of living tissue to electrical stimulation. Most especially, it relates to such a system in which the output pulses used to stimulate the living tissue may be easily varied by the user or by the system itself under suitable program control, depending on the response of the tissue to the waveforms.
2. Description of the Prior Art
The search for chemical substances exhibiting desirable pharmacological properties is complex and never ending. In many instances, this search is hampered by a lack of knowledge of the mechanism at the cellular or even molecular level by which a particular chemical substance produces desirable pharmacological effects.
An approach which has proved to be useful in understanding the mechanism by which a particular pharmacological compound produces its effects has been through applying electrical signals of living tissue and measuring the response of the tissue to these signals both in the absence and presence of a substance under study. This approach is often based on the fact that living tissue produces minute electrical signals which vary as the environment of the tissue is varied. This tissue may also produce observable physiological or biochemical (e.g. metabolic) responses to such electrical stimulation. In order to obtain meaningful information by such electrical stimulus of tissue, it is necessary to change the timing, voltage and current of such signals and study the differences in responses produced by the tissue. It is often then desirable to change the applied signals further on the basis of the responses received to pulses originally applied to the tissue.
Similar considerations apply generally in the biomedical field. Various responses of living tissue to different forms of electrical stimulation are of use in elucidating the mechanisms by which organisms function. When a set of pulses is used to stimulate living tissue, a pattern of responses will often follow an exponential relationship until a steady state response condition is reached. It is often desired to change the set of pulses applied to the tissue at different portions of the exponential response. Because the exact nature of the exponential response is often not known at the time of the beginning of a series of experiments, a system cannot be completely programmed ahead of time to produce the required set of pulses. In many cases, it is desired to stress an organism or tissue with a repeated set of pulses until a steady state response is obtained, apply a different set of pulses to test the organism or tissue at that time, apply new stressing pulses until another steady state response is obtained, apply new testing pulses, and so forth.
Primarily as a result of the need to test performance of electronic circuits, programmable pulse generators for supplying output pulses are known in the art. However, the requirements for such programmable pulse generators in electronic applications are quite different than are required for the study of living tissue. In the electronic field, pulse frequencies in the gigahertz range are common.
While a string of pulses at such high frequency may be fairly complex in the electronic field, once a pulse program for a particular circuit has been established, it usually does not need to be changed. As a result, typical programmable pulse generators for electronic applications rely on a so-called "test can" which includes specialized circuitry for generating a particular string of pulses. With this approach, a new test can must be provided each time the pulse string produced by the pulse generator is to be changed. Alternatively, a pulse string may be changed in prior art test equipment for electronic applications in certain respects by manual operator intervention, such as by setting decade switches and the like.
On the other hand, for biomedical research, lower maximum frequencies in the megahertz (MHz) range are sufficient in a pulse string. However, it is often desired to apply a multiplicity of pulse strings varied in one or more parameters sequentially to a sample. In fact, it is often desired to modify parameters of subsequent pulse strings based on the response received for the sample to earlier pulse strings. Using commercially available pulse generators intended for electronic test applications in the biomedical field is quite time consuming due to the difficulty of making such changes in the pulses produced with them.